In a typical hydraulic control pressure supply system for hydraulic servo brakes and anti-lock brake systems (which are referred to as ALB systems hereinafter), a pressure storage device such as an accumulator is provided at an upstream end of an actuator, and the start and stop control of the pump is carried out according to the level of an inner pressure of the pressure storage device so that a stable actuation pressure may be supplied to the actuator at all times. More specifically, as indicated by the solid line in FIG. 4, the inner pressure of the pressure storage device is maintained within a prescribed range by starting the pump at a certain lower pressure limit which is determined according to the capacity of the actuator and stopping the pump at a certain upper pressure limit which is determined according to the capability of the hydraulic circuit to withstand pressure, the frequency of the operation of the pump and temperature changes.
The prescribed pressure levels serving as the basis for controlling the start and stop operation of the pump are generally fixed values which are based on the maximum capacity of the particular vehicle, Therefore, for instance, in the case of an ALB system, the braking force should be determined according to the travelling speed of the vehicle and is required to be higher as the vehicle speed is increased. But, in reality, the range of the pressure stored in the pressure storage device is fixed, and the pressure existing in the pressure storage device is simply given to the brake circuit without regards to the travelling speed of the vehicle.
In other words, according to the conventional fluid pressure supply system, since the range of the inner pressure of the accumulator is fixed according to the maximum braking force that is required under the severest possible condition, for instance at the time of travelling at high speed, the high pressure corresponding to the maximum capacity of the system is introduced to the brake circuit even when the vehicle is operating under a light condition, for instance at the time of travelling at low speed. Therefore, an excessively high pressure level which is higher than necessary must be finely controlled at the actuator, and the burden of actuator control increases. Furthermore, the fatigue strength must be ensured based on the constant application of the high pressure, and the brake circuit is required to be provided with an excessively high durability.
As an additional factor, since the temperature of the engine room in which the hydraulic pump is located is higher when the vehicle speed is low for a given surrounding temperature, the viscosity of the oil drops when the vehicle speed is low and the oil temperature is high, and the output efficiency of the pump tends to drop. Therefore, the capacity of the pump must be selected to be higher than normally required because the pump is required to produce a high pressure according to the upper limit level determined by a pressure switch even when the vehicle is travelling at low speed.